Automobiles have a continuing need for durable, low-resistance, economical and environment-resistant nesting electrical connectors. By nesting electrical connectors is meant electrical connectors which are characterized by male and female components/contacts which mate with each other such that the female component/contact engages the male component/contact sufficiently to prevent unintentional separation thereof yet permit easy coupling and decoupling thereof by simply pushing or pulling the components apart respectively. Typically, the male component/contact will comprise a blade, pin or the like, and the female component/contact will comprise a complementarily shaped sheath or socket for receiving the blade/pin. The basic requirements for such electrical connectors are that they have (1) a minimal engagement force between the mating contact components/contacts for ease of coupling/decoupling thereof, (2) low contact electrical resistance achievable by utilizing high contact forces, and environment-resistant materials, (3) the capability of multiple couplings/decouplings through the use of wear resistant materials, and (4) prolonged durability (i.e., fretting wear resistance) under micro-rubbing conditions at vehicle engine compartment temperatures. "Micro rubbing" is a condition that occurs in connectors used in vehicular applications such as cars, trucks or the like that undergo considerable vibration, jouncing and thermal cycling while in service which causes the components/contacts of the connector set to minutely move relatively to each other so as to promote fretting wear of the mating surfaces of the contacts.
Copper, its alloys, and stainless steel are often used to form vehicular current-carrying connector components. Copper and its alloys are preferred because of their low electrical resistance. However, copper is prone to oxidation which significantly increases the electrical resistance across the mating contact surfaces (i.e., the contact resistance). Accordingly, various coatings have been proposed for electrical contacts that serve to enhance the electrical conductivity as well as the temperature, chemical and wear resistance of the contact surface. Unfortunately coatings which are effective at low (e.g., ambient) temperature are often ineffective at engine compartment temperatures. A commonly used such coating is electroplated tin. However, tin coatings have a relatively high coefficient of friction making coupling and decoupling of the connector components/contacts difficult. Moreover, tin limits the use of electrical connectors made therewith to temperatures below about 125.degree. C. due to the tendency for interdiffusion of tin and copper which has a deleterious affect on the connector. As engine compartments become more compact, and underhood temperatures commonly exceed 180.degree. C., the relative number of underhood applications that are incompatible with tin-coated contacts is increasing. Electroplated silver has been used as a high temperature coating material for electrical connector applications (e.g, see Cowie et al U.S. Pat. No. 5,139,890). However, to be effective over a prolonged period of use, silver coatings have had to have thicknesses generally greater than about 2.5 microns. In this regard, silver coatings are relatively soft, and hence prone to erosion, particularly at temperatures above about 180.degree. C. Coatings that are too thin can readily wear through and cause early exposure of the underlying metal to oxidation which can produce a high coefficient of friction, and/or a high contact resistance. Multilayer coating systems that employ silver and its alloys as the contact surface for high temperature applications have been proposed. For example, U.S. Pat. No. 4,529,667 to Shiga et al teaches a three layer electroplated coating system comprising a bottom layer of nickel, cobalt, chromium or palladium alloy, an intermediate layer of tin, cadmium, palladium or ruthenium alloy, and a top layer of a silver alloy. Rubbing movement of the connectors components/contacts together can eventually cause thin connector coatings, used heretofore, to fail and dramatically increase the contact resistant between the mating components which can ultimately result in circuit malfunction.
Copending United States patent application Cheng et al U.S. Ser. No.08/543,660 filed Oct. 16, 1995 now U.S. Pat. No. 5,679,471 and assigned to the assignee of the present invention discloses a thin, wear resistant, low friction, corrosion resistant, vapor-deposited silver-nickel coating for the contacts of nesting electrical connectors, which coatings comprise a nanocomposite of nickel-rich and silver-rich phases having a grain size greater than about 5 nanometers.
The present invention is directed to an electrodeposited silver-nickel-carbon nanocomposite coating which has the low friction and corrosion resistance of Cheng et al's Ag--Ni vapor deposited coating, but which is significantly more durable (i.e., more wear resistant) at elevated temperatures than Cheng et al's coating, as well as being easier and cheaper to form.